Many advanced experiments require efficient production of positronium at low kinetic energy. Progress in this field calls for a careful design of the material used to convert positrons into positronium, with regards not only to the chemistry but also to the morphology of the converter. The aim of the present paper is to set ground for future advancements by presenting the current framework of knowledge regarding positronium formation and cooling. The first section addresses positronium formation in bulk solids and at surfaces, with information on yields, energy spectra and angular distributions. The second section discusses the basic principles of collisional cooling in porous solids or in powders and presents a choice of relevant results.